1. Field of the Invention
Embodiments of the present invention relate to control devices for an aircraft having a variable thrust vector.
2. Relevant Background
Soon after the Wright brothers made their first flight in their man-carrying heavier-than-air aircraft in 1903, advances were made in developing a working helicopter. While the feat of developing sufficient thrust to vertically lift a craft off the ground was quickly overcome, the ability to control the airborne craft's flight plagued the development of the helicopter for decades. Even with the rapid advancement of fixed-wing aircraft, vertical flight remained a challenge. It was not until Igor Sikorsky demonstrated his “Vought-Sikorsky 300” in 1939 that a workable solution to vertically controlled flight was realized. Mr. Sikorksy's solution soon superseded others' attempts to solve the control problems plaguing the fledgling helicopter industry and became the model on which modern helicopter aviation is largely based.
Since that time aircraft have essentially been classified as fixed-wing or rotary. Recently, however, a hybrid fixed-wing/rotary aircraft was introduced; it is called the tiltrotor. A tiltrotor aircraft possesses the unique ability to rotate its plane of thrust over a vast range. In one configuration the tiltrotor directs its thrust vertically, much like a helicopter, thus enabling vertical takeoff and landings. In another configuration the thrust component is rotated to a horizontal position giving the aircraft forward speeds generally associated with fixed-wing aircraft. With the introduction of this new type of aircraft came a plurality of control challenges, specifically the challenge of controlling the transition between the craft's functioning as a rotary aircraft to its functioning as a fixed-wing aircraft.
Up until the inception of the tiltrotor, all primary modern flight deck controls had been designed to map to the direction and magnitude of action. For example in a conventional fixed-wing aircraft when a pilot pushes forward on the flight stick, he or she can interpret not only the intended direction of the action, but the magnitude at which the action will occur as well. For this example pushing the stick forward commands the nose of the aircraft to pitch down while pulling back on the stick commands the opposite response, pitching the aircraft up. Furthermore, the more the stick is displaced, the larger the magnitude of the response. The primary flight controls of pitch, roll, yaw, and power are standardized under this construct. This direction and magnitude type mapping has led to a very standardized set of aircraft controls with which pilots are intimately familiar.
Currently two different tiltrotors have been identified for mass production and the control systems on these aircraft are identically mapped with the standard direction and magnitude framework, with one exception: the power control interface. The current power control interface fails to meet the direction component of the direction and magnitude framework in each aircraft during certain phases of flight. In other words the control interface of the tiltrotor aircraft does not consistently convey a sense of the aircraft's reaction based on the input of the flight controls.
The Wright brothers are credited with creating a fully controllable aircraft. Their three-axis control was unique to the time and became the cornerstone of modern aviation. Three-axis control refers to pitch, roll, and yaw, the mainstays of aeronautical control. FIG. 1 depicts a three-axis control orientation 100 as is well known to one skilled in the relative art. Pitch 110 is the rotation along the lateral axis 120; roll 130 is rotation along the longitudinal axis 140; and yaw 150 is rotation along the vertical axis 160. Gliders, jets, helicopters, and even dirigibles utilize this control paradigm. Over the years, the control system of these crafts was standardized into the current configuration now familiar to pilots: the stick (yoke), rudder pedals, and throttle configuration. FIG. 2 shows one version of an aircraft control system utilizing a stick and pedals as is known in the prior art. In the three-axis control configuration 100, yaw 150 is controlled by the rudder pedals 210 and pitch 110 and roll 130 are controlled by the stick 220. In another configuration the stick 220 is replaced by a yoke.
Helicopters create lift by spinning a horizontally-situated rotor to create downward thrust. This configuration also produces torque (the rotational force exerted on a body) which must be countered either by a vertically-situated tail rotor or another counter-rotating rotor (e.g., tandem discs). Through these systems, a helicopter is able to lift off, land, and hover vertically, in addition to moving along a lateral and longitudinal plane. This configuration (with tail or tandem discs) limits the helicopter in its maximum horizontal velocity due to retreating blade stall. Retreating blade stall occurs when the helicopter forward speed (or any direction along the horizontal plane) exceeds the angular velocity at which the blade is moving. When this condition occurs, the retreating blade (relative to the forward velocity of the aircraft) fails to produce lift resulting in an out of control situation.
Helicopters also use a three-axis control configuration in which yaw is controlled by the rudder pedals (tail rotor) and the stick controls roll and pitch of the aircraft. However, unlike a fixed-wing aircraft, the pilot has direct control of the pitch of the wing independent of the pitch of the aircraft. A control known as the collective enables the pilot to command the pitch of the rotors to produce lift. As the pilot pulls up, the collective the blades of the helicopter increase their angle of attach relative to the wind. This pitch is directly related to power; thus as the collective is raised, more power is commanded to the rotors much like a constant speed propeller in fixed-wing aircraft. By comparison, a fixed-wing aircraft uses a separate horizontally moving lever (e.g. throttle) to control thrust/power.
From an aeromechanics perspective, tiltrotors are fully controllable; however, from a human factors perspective, there are inherent control compatibility issues. As described above, helicopter power control interfaces vary greatly from those of their fixed-wing counterparts. Though sticks and rudder pedals are found in both rotary and fixed-wing aircraft, helicopter collective controls and airplane style throttle controls are ill-suited in each of their respective counterparts. This is due to the power interfaces' direction being congruent with the vehicles' respective thrust vector. In a helicopter, the pilot pulls up to go up and pushes down to go down. In a fixed-wing aircraft the pilot pushes forward to accelerate (forward) and pulls back to decelerate (back). Tiltrotors encompass both of these features, but have been built with a single movement, fixed, non-adaptive power inceptor.
Tiltrotors have an added degree of freedom, the nacelle rotation. For the purpose of this application a nacelle, or engine nacelle as it is sometimes referred to herein, means a thrust housing system in which an engine, turbine or other thrust producing device resides. If the aircraft has a fixed-axis power inceptor, there will be incongruence any time the thrust vector is not congruent with the power inceptor's direction of travel. Recall that the typical fixed-wing configuration for power is to advance (move forward) to add power and to retard (move backward) to reduce power. The same is true for a helicopter type of configuration. With the thrust vector positioned forward (0°), the helicopter style of power inceptor control configuration violates the stimulus-response compatibility principle in which the operator's action should move in parallel with the display and his or her mental model. For example, when the pilot desires to add power to accelerate forward (0° nacelle position), he or she is required to pull back and up on the collective in order to do so when in reality this is the reverse action of the net-effect desired. A pilot trained in fixed-wing aircraft learns to pull back on the stick in concert with pulling back on the throttle to land. Imagine the difficulty in retraining such a pilot to pull back on the stick while advancing (pushing down) on the collective in order to land a tiltrotor. Alternatively, a helicopter pilot is familiar with lowering the collective to land. In a tiltrotor having a fixed-wing style power inceptor, moving the power inceptor forward would command full power, the opposite of what is needed to land. This is highly troublesome.
Needed is a control interface design that provides feedback through the pilot's haptic (i.e., touch) senses and requires no mental rotation of the depicted thrust-vector-state information or taxing of the visual senses. These and other control interface challenges are addressed by one or more embodiments of the present invention as is disclosed in detail herein.